New osmosis membrane using transport across nanopores

An improved understanding of how water moves across nanopores could lead to a new type of osmosis membrane. The new membrane purifies water via its geometry rather than the material used, raising the possibility of creating membranes using chlorine-resistant materials.

In a paper published in Nature Nanotechnology¹, Jongho Lee and Rohit Karnik of MIT and Tahar Laoui of the King Fahd University of Petroleum and Minerals in Saudi Arabia, built a proof-of-concept osmotic membrane in which water vapour bubbles are trapped in nanopores. Water is drawn through the membrane by forward osmosis, travelling across the nanobubble by evaporation and condensation. "We expect that this membrane will be useful for rejection of substances like boron that are not easy to reject with current desalination membranes," says Karnik.

Water transport across the nanopores was determined by their shape. In longer pores, transport is only limited by the mass transport of water vapour across the bubble, whereas in shorter pores the water–air interface at the bubble's edge has a more prominent role. The probability that a water molecule striking the liquid surface would condense instead of reflecting back was found to be the limiting factor in transport across short pores.

According to Karnik, these findings may enable us "to more accurately predict behaviour of droplets and bubbles, as well as two-phase flows in porous media at small length scales."

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